Polytetrafluoroethylene (PTFE), commonly available as TEFLON.RTM. TFE fluoropolymer resin (DuPont), while known for its thermal stability, high melting temperature, chemical inertness and lubricity (low coefficient of friction and non-stick character) is also known for its non-melt-fabricability, i.e. it cannot be fabricated by such usual melt-fabrication technique as melt extrusion, including injection molding. Instead, this resin is fabricated by such non-melt flow techniques as paste extrusion (fine powder type of PTFE) and compression molding (granular type of PTFE) into strong articles, the strength of which can be augmented by sintering. The non-melt fabricability of the PTFE resin arises from the fact that it does not flow in the molten state, whereby it has a melt viscosity of at least 1.times.10.sup.8 Pa.multidot.s at 380.degree. C. The terms "polytetrafluoroethylenel" and "PTFE" used in the literature refers to this non-melt flowable, non-melt-fabricable PTFE. For clarity in the discussion to follow, however, this PTFE will be referred to as high MV (melt viscosity) PTFE.
Low MV PTFE is also commercially available and used in small amounts, relative to the consumption of high MV PTFE, the low MV PTFE having an MV of 50 to 1.times.10.sup.5 Pa.multidot.s at 372.degree. C. and available for example as ZONYL.RTM. fluoroadditive (DuPont). The low MV PTFE can be obtained by irradiation degradation of the high MV PTFE to reduce the molecular weight or directly by polymerization technique such as disclosed in U.S. Pat. No. 5,223,343 (Example 1). In either case, the number average molecular weight (Mn) of the low MV PTFE is from 50,000 to 700,000, as compared to the molecular weight (Mn) of high MV PTFE which is at least 2,000,000. The low MV PTFE has the thermal stability, chemical inertness, lubricity, and high melting temperature similar the high melt viscosity PTFE. The much lower MV of this PTFE makes the PTFE melt flowable, as indicated by its melt viscosity mentioned above, but the much lower molecular weight of this PTFE deprives it of strength, whereby articles molded from this low MV PTFE by melt extrusion break upon handling. For example, the beading extruded in the MV determination breaks upon the slightest flexing, and tensile testing samples formed by injection molding break upon mere clamping in the tensile testing machine, whereby the resin has no measureable tensile strength. Thus, although melt flowable, the low MV PTFE is not melt fabricable. This has relegated the utility of the low MV PTFE to such uses as additives in liquid lubricants and in supported coatings. This low MV PTFE is often distinguished from the PTFE described above as the high MV PTFE by being called PTFE micropowder.
There is a need to find greater utility of the low MV PTFE.